1. Field of the Invention
The present invention relates generally to a point detecting device and a method for providing locational information such as coordinate information which is indicated by a pointing device. The present invention especially relates to a point detecting device adapted for a large scaled system which is required to be operated at low cost and at high accuracy.
2. Description of the Prior Art
Conventionally, in various computer-aided working fields such as computer graphics and computer aided design, it has been common knowledge to use various point detecting devices which analyze electrical signals generated by electro-magnetic means energized by a point detecting device to provide locational information pointed out by the pointing device.
A typical example of conventionally used point detecting devices is composed of a sensor unit including a plurality of loop coils which are arranged in parallel with each other in the direction to be detected, and a pointing device including at least one tuning circuit. Any one of the loop coils is selected and energized by an AC signal to generate an electromagnetic wave so that the tuning circuit included in the pointing device is energized by the electromagnetic wave. Since this energized tuning circuit also generates electromagnetic wave, the above selected loop coil receives this electromagnetic wave from the tuning circuit thereby resulting in induced voltage. This induced voltage is detected as a first received signal. This operation will be repeated by succeedingly selecting one by one from the loop coil series to detect amplitude and phase angle of the respective received signals. The data thus obtained is calculated to provide locational information such as the coordinate value representing the point pointed by the pointing device.
In order to detect the amplitude and phase angle of respective received signals, there are two commonly known methods one of which is based on analog phase detection as referred to by Japanese Patent Application Open to Public Inspection No. 63-70326 (1988) and the other is based on high speed AD transform and discrete Fourier-transform as referred to by Japanese Patent Application Open to Public Inspection No. 3-147012 (1991).
The above described sensor unit will be further described in detail. As discussed above, conventional sensor units include a plurality of loop coils each one of which is formed in a substantially rectangular shape (winding number of the coil is not limited), which are arranged in parallel in the position detecting direction, for example, X-axis direction. In order to perform point-detection in dual axes for example X and Y axes, such the loop coil configuration is respectively arranged in X and Y axes directions. Each one of the loop coils is associated with a pair of a signal transmitter and a signal receiver. The transmitter and receiver pair are located at either side of the longitudinal ends of the loop coil. According to this configuration, if the transmitter is transmitting a signal to the associated loop coil and a pointing device is located on this loop coil, then the pointing device will generate a signal due to electromagnetic mutual function between the pointing device and the transmitted signal. Then this signal from the pointing device will be detected by the signal receiver.
In alternative configuration, either the transmitter or receiver only is arranged to the loop coil side.
In the case of only the transmitter, a pointing device is provided with a receiver mechanism and the transmitter is arranged at one of the longitudinal ends of the loop coil. According to this configuration, if the transmitter is transmitting a signal to the associated loop coil and the pointing device is located on this loop coil, then the pointing device will generate a signal due to the electromagnetic mutual function between the pointing device and the transmitted signal. Then this signal from the pointing device will be detected by the receiver mechanism included in the pointing device.
In the case of only the receiver, a pointing device is provided with a transmitter mechanism and the receiver is arranged at one of the longitudinal ends of the loop coil. According to this configuration, if the transmitter mechanism of the pointing device is transmitting a signal to the coil of the pointing device, then the pointing device will generate a signal due to electromagnetic mutual function between the pointing device and the loop coil under the pointing device. Then this signal from the pointing device will be detected by the receiver associated with this loop coil.
In addition to the above described configuration, Japanese Patent Application Open to Public Inspection No. 5-241722 (1993) discloses an example of point detecting device which makes a pointing device generate self vibration by forming a normal feedback loop of an amplifier when this pointing device is electromagnetically coupled with two coupling means which are prohibited from coupling with each other, and thus the self vibration is detected as a point signal of the pointing device.
As practical examples of the above described point detecting devices, there is known a tablet and a touch display used as an input device for various computer systems. Their sensor section generally does not have a large area, rather than the range from type B-5 to A-4 on size of paper sheet in Japanese Industrial Standard. Furthermore, there are a few examples of a large scaled point detecting device such as an electronic black board whose sensor requires the large area as well as from type A-2 to A-0.
Referring to FIG. 22, there is shown an example of signal detecting operation in a conventional manner. This drawing represents that a signal Vc is induced by the electromagnetic function between a pointing device at a point P and a loop coil, and the signal Vc is detected as a signal V by a receiver arranged at one end O of the loop coil. Generally, in point detecting devices constituted in this manner, each loop coil has a longer loop elongated along one axis. This type of loop coil can be treated as a uniform distributed constant circuit. When electric energy such as voltage or current is transmitted through a uniform distributed constant circuit, the intensity (level) of this electric energy is gradually decreased and its phase is simultaneously changed, as the distance "d" between the receiver and the pointing device is prolonged. This relation is represented by the following equation; ##EQU1##
In the equation (1), R[.OMEGA./m], L[H/m], C[F/m], and G[S/m]represent distributed constants of this loop coil, and depend on the material of loop coil. .omega.=2.pi.f; f is frequency [Hz].
In the case shown in FIG. 22, the received signal V is represented by the following equation (2) according to the arithmetic operation of distributed constant circuit; EQU V=A.multidot.exp (-2.alpha.d).multidot.exp (-2j.beta.d) (2)
In the equation (2), "A" represents a constant including conversion constant during transmitting and receiving operation; "exp (-2.alpha.d)" represents the attenuated amount; "exp ((-2j.beta.d) represents the phase delayed amount; and "d" represents the distance between O and P. As is clear from the equation (2), the level and phase of the signal received by the receiver depend on the signal received position on the loop coil. The level will be attenuated and the phase will be delayed as the distance between the signal received position and the receiver is prolonged. FIG. 23(a) and 23(b) show graphical representations with respect to changes in the level and phase of the received signal by the receiver respectively. The data shown in FIG. 23(a) represents the result from the receiver positioned at the left end of the loop coil as well as FIG. 22. As the sensing area of a point detecting device is larger, the detected signal tends to be easily affected by the distance "d". Furthermore, since the distributed constant of one loop coil is not always equivalent to that of the other, the difference between two or more loop coil is naturally amplified in response to the distance "d". FIG. 23(a) and (b) show the amplitude difference by ".DELTA.L". FIG. 23(b) shows an alternative case where the receiver is arranged at the right side of the loop coil. Thus the change in the level and phase of the received signal appears symmetrically as in the case of FIG. 23(a).
The same phenomenon appears when a signal is transmitted from the position O to make electromagnetic function with the pointing device at point P.
As discussed above, the level attenuation and phase delay depending on the distance between the pointing device and the signal transmitter or receiver, and the variation in the received or transmitted signal generated between two or more loop coils will result in arithmetic errors. These errors will be generated more markedly as the length of the loop coil extends.
Typically, a loop coil is manufactured by forming a loop shape conductive pattern on a substrate in a vacuum evaporation or printing manner. Ordinary sized point detecting devices employ loop coils made of copper or silver formed on a substrate by printing. However, such metal deposited loop coils provide relatively high impedance which result in too large a propagation constant to use normally. Therefore, large scaled point detecting devices employ loop coils made of aluminum providing a low impedance by means of vacuum evaporation. Such manufactured loop coils are or remarkably high cost.